1. Field of the Invention
The present invention relates to an electroconductive rubber composition for electrophotographic apparatuses suitable for forming a dielectric layer of a charging roller for use in electrophotographic apparatuses and to a charging roller for use in electrophotographic apparatuses manufactured using the electroconductive rubber composition.
2. Description of the Related Art
In recent years, there has been increasing use of electrophotographic apparatuses, such as electrophotographic copying machines, printers, and facsimile machines. In general, electrophotographic apparatuses include a photoconductive drum and various electroconductive rollers around the photoconductive drum, such as a charging roller, a developing roller, a transfer roller, and a toner-supply roller.
In reproduction or printing with an electrophotographic apparatus of this type, an electrostatic latent image of an original image is formed on a photoconductive drum. The electrostatic latent image is then coated with toner to form a toner image. The toner image is finally transferred to transfer paper. In order to form the electrostatic latent image, a charging roller is used to electrify the photoconductive drum. Recently, photoconductive drums have often been electrified by contact electrification, in which a charging roller is directly brought into contact with the surface of a photoconductive drum.
An exemplary charging roller includes a shaft, such as an electroconductive shaft, an elastomeric layer (base layer) around the shaft, and if necessary a resistance-adjusting layer (intermediate layer) or a surface protective layer (surface layer) around the base layer. The elastomeric layer (base layer) and the resistance-adjusting layer (intermediate layer) are formed of an electroconductive rubber composition.
In order to efficiently and uniformly transfer electric charges to toner and the photoconductive drum, the rubber composition for use in the elastomeric layer (base layer) or the resistance-adjusting layer (intermediate layer) should have low electrical resistance and electrical uniformity. Since electrophotographic apparatuses of this type can be used at various temperatures and humidity, the electrophotographic apparatuses should be electrically stable against any environmental change.
Ion carriers that can be dispersed in a rubber composition at the molecular level are effective to achieve electrical uniformity of the rubber composition. Examples of such ion carriers include ionic liquids, quaternary ammonium salts, and phosphonium salts.
Japanese Patent No. 4,392,745 discloses a rubber composition containing an imidazolium or pyridinium ionic liquid as an ion carrier. Japanese Unexamined Patent Application Publication No. 2002-132,020 discloses an electroconductive roller that includes an elastic layer around an electroconductive shaft, wherein the elastic layer is formed of a rubber composition containing a quaternary ammonium salt as an ion carrier. Japanese Unexamined Patent Application Publication No. 2001-279,104 discloses an elastic composition containing a tetraethylphosphonium salt, a triethylbenzylphosphonium salt, or a tetramethylphosphonium salt as an ion carrier.
As described in Japanese Patent No. 4,392,745 and Japanese Unexamined Patent Application Publication No. 2002-132,020, however, it was found that an imidazolium, pyridinium, or ammonium ion carrier bled on the surface of a charging roller in an electricity breakdown test. The ion carriers bleeding on the charging roller may pollute a photoconductive drum in contact with the charging roller and lower the image quality.
It was also found that a phosphonium salt described in Japanese Unexamined Patent Application Publication No. 2001-279,104 used as an ion carrier could not sufficiently reduce the electrical resistance of the rubber composition and could not achieve low electrical resistance required for the charging roller. It was also found that there was a large difference between the electrical resistances before and after the electricity breakdown test of the charging roller.